1. Field of the Invention
The present invention relates to a semiconductor laser and a method for manufacturing the same.
2. Description of the Prior Art
Semiconductor lasers have been highlighted as a light emitting source for optical fiber communication and optical information processing since continuous oscillation at room temperature of GaAlAs based semiconductor lasers was reported in 1970. It is not too much to say that such semiconductor lasers have been developed according to technical requirements in optical fiber communication.
Following the development of GaAlAs based semiconductor layers oscillating at a wavelength range of 0.7 .mu.m to 0.9 .mu.m, the development of InGaAsP based semiconductor lasers oscillating at a long wavelength range has been also promoted according to an increase in the minimum loss wavelength of optical fibers, for example, by 1.1 .mu.m to 1.6 .mu.m.
In addition to such semiconductor lasers for communication, on the other hand, another type of semiconductor lasers have been manufactured in mass, which oscillate at 0.7 .mu.m, for processing optical information of laser printers, compact discs and video optical discs. Applications of various industrial semiconductor lasers have also extended rapidly.
Semiconductor lasers have a higher efficiency over other types of lasers and achieve modulation at high speed. They are also advantageous by virtue of their very miniature constructions.
The semiconductor lasers can have a wavelength of emitted light selected variously within a wide wavelength range from the wavelength of visible light to the wavelength of far infrared ray. Their life is also assured up to several decades.
Recently, the disadvantage of the semiconductor lasers having poor interference over other types of lasers has been compensated and the optical output of semiconductor lasers is also on an increasing trend. In this regard, the semiconductor lasers are expected to be substituted for gas lasers and solid state lasers in a field involving no requirement of high energy.
Now, the operational principle of the above-mentioned semiconductor lasers will be described and the construction of a conventional semiconductor laser will be also described.
Semiconductor lasers are made of a compound semiconductor such as GaAs or InP which has a direct transition type energy band structure. As voltage is forwardly applied to a P-N junction of such a semiconductor, current flows through the semiconductor. This current flow causes electrons in the N-region and holes in the P-region to flow toward the opposite regions and to be recoupled together and thus emit light.
At a small quantity of current flow, the recoupling of electrons and holes are achieved irregularly. As a result, any induced emission of light required for operating the semiconductor laser is not achieved since occurring optical waves have no correlation with one another. At a large quantity of current flow, an inverted electron distribution is formed near the P-N junction, as shown in FIG. 1. The inverted electron distribution means that more electrons at a lower energy level are distributed, over a higher energy level. At such an inverted electron distribution, light is inducibly emitted by virtue of the electron-hole recoupling. In this case, the region at which the inducible emission of light caused by the inverted electron distribution is called an active region or a gain region.
As a gain larger than a loss of a resonator is generated upon an increase of current applied, the laser is oscillated. The current providing the gain required for oscillating the laser is called a threshold current which is the important dimension for determining the performance of the usable condition of semiconductor laser.
In a semiconductor laser, the resonator for obtaining the oscillation of optical waves uses mainly the crystal sectional surfaces (namely, the surfaces of cleavage) of the semiconductor itself.
A discussion will be now made for an efficient structure capable of reducing the threshold current in the semiconductor laser. If the recoupling of carriers (namely, electrons and holes) is achieved at a region involving no inverted electron distribution, that is, outwardly of a gain region, the light emitted due to the recoupling can not contribute to the inducible emission.
In such a double hetero (DH) structure, an active layer (GaAs) having a small energy gap is interposed between clad layers (GaAlAs) having a large energy gap. A semiconductor laser having such a DH structure is a simple diode in which its one hetero-junction is a P-N junction. As current flows forwardly through the diode, the holes in the N-type clad layer flow into and are then implanted in the active region.
Since the active layer has a small band gap as shown in FIG. 2, the clad layers disposed at opposite surfaces of the active layer form energy barriers which function to restrain implanted carriers in the active region. Accordingly, the density of carriers in the thin active region is very high and the recoupling of carriers for emitting light is mainly achieved in the active layer.
The refractive index of GaAs constituting the active layer is higher than that of GaAlAs constituting the clad layers. Light has a property of concentrating on the region having a large refractive index. In the DH structure, accordingly, light is focused on the active layer, so that densities of carriers and optical waves in the active layer are very high, thereby enabling the threshold current to be reduced.
Furthermore, the threshold current can be further lowered by restraining the carriers and the optical waves in a direction perpendicular to the active layer in the narrow active region. To this end, a metal electrode having a narrow strip shape is formed as shown in FIG. 2, to control the flow of current spatially. As shown in FIG. 2, opposite side surfaces of the metal electrode are formed roughly, so as to prevent the light concentration. Such a structure is called a strip type hetero structure. It is often called a gain transmission type structure since light is guided to a gain region in which the density of carriers is high.
In addition, there is a method for restraining optical waves in a direction parallel to the active layer. In this connection, FIG. 3 shows a buried type hetero structure which is commonly used in communication lasers. As shown in FIG. 3, the structure has a shape that a GaAs active layer is surrounded at its opposite sides by N-type GaAlAs layers.
As above-mentioned, the GaAs layer forms a waveguide path since it is surrounded at its upper, lower, left and right portions by the GaAlAs layers having a refractive index lower than that of the GaAs layer.
As shown in FIG. 3, opposite side surfaces of the waveguide are formed roughly, so as to prevent the light concentration. This type of waveguide is called the refractive index waveguide type.
Such a DH structure has an advantage of a low threshold current. Also, it has a stable oscillation transverse mode characteristic and is advantageous for communication and a information processing.
For using semiconductor lasers in signal processing, a switch characteristic for intermittently controlling the emission of laser is generally required.
In continuous oscillation type conventional semiconductor lasers shown in FIGS. 2 and 3, the supplying of current a to semiconductor laser is switched for switching the laser beam emission. Such a method is difficult to process signals having a frequency of several MHZ or greater, due to the limitation of a drive circuit of the semiconductor laser.
In this connection, a semiconductor layer having a structure shown in FIG. 4 has been proposed by Y. Kan et al in 1986 (IEEE J. QUANTUM ELECTRON, QE-22, 1837, 1986). In such a semiconductor, an electric field is applied to an active layer which is, in turn, excited by external light, for example, He--Ne laser beams, thereby causing the laser to be oscillated.
At this time, the oscillated laser is sensitively affected by the applied electric field and thereby does not oscillate at a bias voltage V1 of -5 V. Accordingly, when charges are filled in a hetero junction, upon applying current to the semiconductor laser, the frequencies of processible signals oscillated are limited to several MHZ, as shown in FIGS. 2 and 3. However, when the oscillation of laser is controlled by the electric field, it is possible to process signals having a frequency of up to several Tera HRZ.
The structure and operation of the semiconductor laser proposed by Y. Kan et al will now be described in conjunction with FIGS. 4 to 6.
FIG. 4 is a sectional view of the structure of a semiconductor laser. As shown in FIG. 4, the semiconductor laser comprises a semitransparent electrodes 1 as an uppermost layer and an Au/Cr electrode 2 as a lowermost layer. These layers serve to cause a current flow into the semiconductor laser. The semiconductor laser has at its middle portion a GaAs active layer 3 having a thickness of 120 .ANG.. Over upper and lower surfaces of the active layer, a pair of GaAlAs clad layers 4 and 5 having a thickness of 0.3 .mu.m are formed, respectively. Between the semitransparent electrode 1 and the GaAlAs clad layer 4, a N-GaAlAs layer 6 is formed which has an impurity concentration of 3.times.10.sup.17 cm.sup.-3 and a thickness of 1.5 .mu.m. On the other hand, a P-GaAlAs layer 7 having an impurity concentration of 3.times.10.sup.8 cm.sup.-3 and a thickness of 2 .mu.m is formed between the Au/Cr electrode 2 and the GaAlAs layer 5. Finally, a GaAs substrate 8 is formed at a predetermined portion between the semitransparent electrode 1 and the N-GaAlAs layer 6.
FIG. 6 shows an energy band structure corresponding to the structure of FIG. 4. As an electric field is applied to the semiconductor laser, electrons presented in a conduction band and holes presented in a valence band move opposite directions with respect to the electric field, thereby causing the recoupling rate therebetween to be reduced. As a result, the oscillation is stopped and the laser emission is cut off. That is, the oscillation is stopped when the bias voltage V1 is -5 V, as shown in FIG. 5.
When the electric field, namely, the bias voltage V1 disappears, the electrons and holes return to their original positions. Accordingly, the oscillation occurs again and thus laser beams are emitted.
As apparent from the above description, Kan's technique makes it possible to switch rapidly the emission of laser beams from the semiconductor laser by oscillating the laser at its active layer using He--Ne laser beams and applying an electric field in a direction perpendicular to the active layer or removing the electric field.
In all of the above-mentioned conventional semiconductor lasers, however, the oscillation is achieved by crystal sectional surfaces (namely, the surfaces of cleavage) formed perpendicular to the active layer and the emission of laser beams is achieved in a direction parallel to the surface of cleavage. As a result, the conventional semiconductor lasers can be suitably used only for the manufacture of unit semiconductor lasers. They are unsuitable for the manufacture of semiconductor lasers of an integrated type in which unit semiconductor lasers are arranged in two dimensions. In these semiconductor lasers, consequently, it is impossible to process simultaneously a plurality of signals such as video signals.